After the invention of the light bulb, many innovators devised light sources, and luminaires for supporting and operating them, that produced light which radiated from an axis, rather than just a point. A variety of means for diffusing, reflecting, or altering the light from these sources were employed. Linear incandescent lamps persisted, but fluorescent tube lighting largely took over linear applications. Light emitting diodes used for indicators later were also used for illumination, including linearly.
Numerous types of housings supported the light source and energizing components. The fixture housing, type of light source, and light modifiers were wed by design. Energizing components had to be serviced in place, even when the luminaire was far from reach, or not readily accessible. Housings were bulky, heavy, and difficult to mount or install. Long rows of linear lighting made by assembling individual luminaire housings typically were crooked or challenged to remain straight.
A plethora of light modifying coverings mostly molded from plastic, or crafted from metal or wood, often had non-obvious means for removing and replacing coverings. It was difficult to replace lamps, and breakage of the cover was not uncommon. Coves or valences made for indirect lighting, hid the luminaires and lamps entirely from customary views, but this also made the lighting installation difficult and time consuming. Luminaires for indirect lighting that were suspended below a ceiling were arranged with components above the housing, limiting access to them.